1. Field of the Invention
This invention relates to an element for use in detecting an oxygen concentration of gases, and a process for producing the same.
2. Description of the Prior Art
Recently, many attempts have been proposed to provide exhaust gas emission control systems for an automotive internal combustion engine for the purposes of improving the fuel consumption and reducing the amounts of harmful constituents of exhaust gases. It has thus been proposed to detect the oxygen concentration of the exhaust gases and then adjust the flow rate of air or fuel to be fed into the internal combustion engine according to a signal thus detected. The present invention is directed to providing an element for use in detecting the concentration of oxygen contained in gases, such as exhaust gases from an automotive engine.
Various kinds of oxygen concentration detecting elements have been proposed. Among these, a sintered body of an oxide-semiconductor material, such as titanium oxide provides the advantages of simple construction and compact size. However, these detecting elements without exception suffer from a disadvantage in that the detecting elements of oxide-semiconductor type fail to function properly, unless the gases measured are maintained at temperatures as high as about 800.degree. C. (See FIG. 2 showing a comparative example).
More particularly, in case an oxygen concentration detecting element is used in an exhaust-gas emission control system of an internal combustion engine of the type described above, it is mandatory that the resistance of the detecting element should sharply change in an exhaust gas condition corresponding to a stoichiometric air to fuel ratio, as shown by curve 1 in FIG. 2. The prior art detecting elements of the oxide-semiconductor type merely exhibit slow changes in their resistances for exhaust gases at 700.degree. C. as shown by curve 2, and at 400.degree. C., as shown by curve 3 in FIG. 2. In other words, the use of prior art detecting elements is impossible over a wide temperature range.
Further, the oxygen detecting element should endure both the oxidizing and the reductive atmospheres in the exhaust gas and keep its performance for a long time at elevated temperatures such as above 400.degree. C.
To cope with this problem, there has been proposed an oxide-semiconductor type detecting element, in which the surface portion thereof is first permeated with chloroplatinic acid, and then heated, thereby obtaining a platinum-supporting detecting element, in which platinum is supported or carried on the surface portion of the element. The platinum-supporting detecting element, as shown as a comparative example (FIG. 3) in Example 1, enables the use of a wide temperature range but fails to provide practical use during long periods of time, i.e., fails to provide desired durability. (FIG. 5, comparative example, shown in Example 1.)
In standard use, the oxygen concentration detecting element is placed in gases with atmospheric conditions changing alternately from a reductive atmosphere of low oxygen concentration to a highly oxidative atmosphere of high oxygen concentration. It may happen that the oxygen concentration detecting element is exposed to a reductive atmosphere for a long period of time, after which the atmosphere is suddenly replaced by an oxidizing atmosphere. It is necessary in this case as well, that the detecting element rapidly respond or follow the aforesaid change in atmosphere so as to exhibit a change in resistance commensurate with the varying oxygen concentrations. The prior art oxide-semiconductor type detecting elements and their improvements, i.e., platinum-supporting detecting elements, all fail to follow or respond to a rapid or sharp change in oxygen concentration, thus resulting in difficult reproducibility of the resistance. In other words, a hysteresis characteristic results.